Internal compiler error- stage DEFINE symbol

Hello...

Im implementing interface concept in asp.net web application to insert city details to database table. Im getting error as "Internal Compiler Error: stage 'DEFINE'" i couldn't able to access the interface and the class which has been implemented by that interface in asp webform...!

aspx.cs code:-

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Data;

using Interface_Project.Presenter.PresenterAdmin;

using Interface_Project.Model.ModelAdmin;

namespace Interface_Project

{

public partial class ManageCity : System.Web.UI.Page

{

ModelCityDetails modelObjCity = null;

protected void Page_Load(object sender, EventArgs e)

{

modelObjCity = new ModelCityDetails(this);

}

protected void btnAdd_Click(object sender, EventArgs e)

{

if(modelObjCity.insertCityDetails())

{

ClientScript.RegisterStartupScript(this.GetType(),"alert","");

}

else

ClientScript.RegisterStartupScript(this.GetType(),"alert","");

}

}

public partial class ManageCity : Interface_Project.Presenter.PresenterAdmin.ManageCity.IManageCity

{

public int cityid

{

get

{

return 0;

//return int.Parse(hdnCityId.Value);

}

set

{

//hdnCityId.Value = value.ToString();

}

}

public string cityname

{

get

{

return txtCity.Text;

}

set

{

txtCity.Text = value;

}

}

}

}

Any help appreciated...! Thank you...!